Aluminum-based,heat treatable alloy

ABSTRACT

A HIGH STRENGTH, HEAT TREATABLE ALLOY CONSISTING ESSENTIALLY OF FROM ABOUT 0.6 TO 3.6% MG; ABOUT 0.5 TO 1.5% SI AND ABOUT 0.2 TO 0.5% BE IS PROVIDED WHICH IS AGE HARDENABLE AND DISPERSION HARDENABLE.

March 30, 1971 I B. GRIFFITHS 3,573,035

ALUMINUM-BASED, HEAT TREATABLE ALLOY Filed Jan. 24, 1969 2 Sheets-Sheet 1 EFFECT OF Be UPON FLOW CHARACTERISTICS OF A\-Mg Si ALLOYS AT ROOM TEMPERATURE SLOPE AT 0.002 STRAIN =I.4 X IO PSI (IO Kg-mm' Al- I [Mg-0.9% Si w SLOPE AT 0.002 STRAIN F143 17 I 5 =45 x I PSI(3.2 x lO Kg-mm' LL] 3 '-1-YIELD POINT (NORMALISED) l I I I l l 0002 TRuE STRAIN EI E 3 5,I2- -I.5 X 0.2% OFFSET YIELD N m I E E 3g I0- d I- x w 9- O MICROYIELD sTREss 8 .I -|.O J m E m PM. 2 E 2 It 4 5 0 2 g -0.5 2 O 3 2 I INVENTOR 0 I I LEONARD B. GRIFFITHS 0 I0 BY J AGEING TIME IN MINUTES (1/- 7 ATTORNEY March 30, 1971 Filed Jan. 24, 1969 2 Sheets-Sheet 2 i 0.2% OFFSET YIEL x |2- N E E (n E FIG 5' Q '1 j MICROYIELD STRESS (/7 a 5 E E m 5 3 E 5; 5? 2 2 O O I I I I I IO 20 3o 40 50 6O AGEING TIME IN MINUTES Be-BEARING ALLOY- NO Be-- E FINE. 0 X

(I) V) DJ E (/7 INVENTOR LEONARD B. GRIFFITHS ATTORNEY United States Patent US. Cl. 75-147 25 Claims ABSTRACT OF THE DISCLOSURE A high strength, heat treatable alloy consisting essentially of from about 0.6 to 3.6% Mg; about 0.5 to 1.5% Si and about 0.2 to 0.5% Be is provided which is age hardenable and dispersion hardenable.

This application is a continuation-in-part of application Ser. No. 667,910 filed Sept. 15, 1967.

There are two primary methods by which metals may be strengthened by incorporation of a second phase. These are known as precipitation and dispersion strengthening. The former is a solid state, reversible (under heat treatment) process and may or may not involve development of lattice coherency strain.

The latter, in general, is not reversible by heat treatment and normally involves the incorporation of an inert phase, present as particles of generally less than 0.5 in the matrix. Generally dispersion strengthening is produced by powder metallurgy.

It is an object of the present invention to provide an aluminum alloy containing Mg, Si and Be which is strengthened by age hardening, wherein the Be causes changes in the precipitation mode yielding an alloy which behaves similarly to a dispersion strengthened material.

It is another object of the present invention to provide an aluminum alloy containing Mg, Si and Be which is strengthened by dispersion hardening.

It is another object of the present invention to provide an aluminum alloy having high yield strength, high tensile strength and good ductility.

It is another object of the present invention to provide a high strength aluminum base alloy containing Be.

A further object of the present invention is to provide a ductile, high strength alloy of aluminum-beryllium-magnesium-silicon.

Another object of the present invention is to provide a desirably ductile alloy of beryllium-aluminum-magnesium-silicon wherein the alloy consists of either an excess of silicon with respect to magnesium to a degree whereby the alloy may be categorized as silicon rich or an excess of magnesium with respect to silicon to a degree whereby the alloy may be categorized as magnesium rich.

Other objects will appear from the following description and drawings.

In the drawings:

FIG. 1 is a graphic showing the stress-strain relationship of an aluminum 1%magnesium 0.9%-silicon alloy and an aluminum l%magnesium 0.9%silicon 0.5%beryllium alloy.

FIG. 2 is a plot of the variation of 0.2% ofi'set yield stress and the micro-yield stress a function of ageing time at 240 C. for a 1% Mg. 1% Si, 0.4% Be-bearing Al alloy.

FIG. 3 is a plot of the variation of 0.2% offset yield stress and the micro-yield stress a function of ageing time at 240 C. for .9% Mg, .9% Si, Be-free A1 alloy.

FIG. 4 is a plot showing the variation of the initial portions of the stress strain curves of Al-Mg-Si and Al- Mg-Si-Be alloys with ageing time.

'ice

The composition of the alloy of the present invention consists essentially of about 0.6 to 3.6% by weight Mg, from about 0.5 to 1.5% Si, and about 0.2 to 0.5% Be, the balance essentially aluminum.

In the equilibrium state the phases present in the quaternary Al, Mg, Si and Be of the present invention alloy are: (1) Al solid solution, (2) an intermediate phase of composition approximately Al Mg (3) Mg Si.

Mg Si crystallizes from the melt during solidification. The distribution within the alloy is a function only of the rate of solidification. Therefore, rapid cooling, for example 3 to 8 C./sec. through the liquid-solid range is necessary in order that a tfine dispersion of Mg Si is obtained. In this way some degree of dispersion strengthening may be obtained in the cast product.

The most economical manufacturing technique for the alloy under consideration is continuous casting using chill plate techniques. Such processes are well known in the commercial foundry field. In this area there is an advantage in that cast dispersion strengthened alloys are not generally available. Powder metallurgy is normally employed in order to obtain dispersion strengthened alloys.

Precipitation of the intermediate phase Al Mg is a solid state process. In order to develop optimum properties r a homogenized alloy is heated to a temperature where all of the magnesium (i.e. all but that bound up as Mg Si) is in solid solution in aluminum. Temperatures within the range of about 375 to 500 C., (for example about 400 C.) are sufficiently high to facilitate solutionizing in an alloy containing 3.6% by weight of Mg. The alloy is then quenched into a suitable medium (e.g. water or oil) at temperatures below about C. This process yields a two phase alloy, Mg Si plus Al supersaturated with Mg.

Subsequent heat treatment at a temperature above 100 C. but below about 400 C. leads to precipitation of Al Mg A temperature of about ZOO-220 C. has been used which leads to peak strength after a holding time of 14 hours.

As is common to all precipitation hardened alloys, the strength level achieved is a sensitive function of the distribution and size of the precipitated phase. This in turn is a function of the precise heat treatment conditions.

Within the ranges of proportions of elements contributing to the ductility and strength of the alloy two different species arise. This arises because magnesium combines with silicon to form Mg Si more easily than it combines with aluminum to form Al Mg Thus whenever the atomic ratio of magnesium to silicon present in the composite is about 2:1 or less, no significant amount of Al Mg is present thereby providing an alloy that may be classified as silicon rich. The properties of the Si rich alloy are as follows:

Minimum 0. 2%

Minimum percent elongation Alloy 1% Mg, 1% Si. Aged 1% Mg, 1% Si, 0.5% Aged Condition magnesium to silicon not exceed 2:1. The atomic ratio of 2:1 is equivalent to a weight ratio of about 1.72: 1.

It should be kept in mind that small proportions of beryllium, about 0.2 to 0.5% by weight are present in the alloy. When magnesium is present in an amount exceeding a 2:1 atomic ratio to silicon, the alloy system is either Al(ss) +Al Mg or Al(ss) +Al Mg +Mg Si (The notation (ss) means solid solution.) However, when the magnesium is present in an amount of about 2:1 or less atomic ratio to silicon the alloy system is either Al (ss) +Mg Si or Al(ss) +Mg Si+Si (to the extent that silicon is present in excess of the stoichiometric amount required for Mg Si).

The excess of silicon included is desirably less than about 1.0%, by weight and preferably less than 0.5 by weight of the alloy constituents. Moreover, it is preferred that each of the additions of magnesium and of silicon not exceed more than about 1.0% by weight of the constituents of the alloy, but have at least 0.6% Mg and 0.5% Si.

The previously mentioned small amount of beryllium present in the alloy, up to about 0.5 by weight of the alloy, has a very important influence upon the solid state precipitation hardening process in the alloy. The influence of beryllium is well developed in alloys of Al+Mg Si (with or without excess Si). In a pure Al-Mg-Si alloy, for example Al1% wt. Mg-0.5% wt. Si, the Mg Si phase precipitates from quenched solid solution via an intermediate, partially coherent, stage. Establishment of this quasi-equilibrium configuration is dependent upon the excess vacant lattice sites which are present in the asquenched structure. However, with beryllium also in solid solution, this stage does not occur to the same extent (because the beryllium atoms associate with the vacant lattice sites) and the equilibrium Mg Si structure forms directly. Further, precipitation is found to be heterogeneous occurring at the sites of dislocations. These features have been found to influence the work hardening rate more than the (engineering) flow stress. In other words the behavior is closer to a dispersion rather than a precipitation hardened alloy.

The rate of work hardening is very much higher especially in the early stages of plastic yielding. FIG. 4 serves to illustrate these effects.

The alloy can be used, for example, as a structural member at ordinary temperatures or as a rivet-rod product where cold upsetting is employed to form the rivet in situ. Ageing may then be performed after cold upsetting if desired.

The alloy could also find application as a structural member in environmental temperatures up to 150 C.

It is to be understood that other additions may be made to the alloy of the present invention which do adversely effect the novel characteristics thereof.

It is apparent from comparing FIG. 2 with FIG. 3 that the beryllium addition results in some increased strength in the age hardened condition, although a slightly longer ageing time is required to obtain the greater strength.

As can be seen from FIG. 4 if work hardening is carried out after ageing, the beryllium alloys give substantially higher strengths than alloys not containing beryllium, and the strength gain increases with the amount of work hardening.

EXAMPLE I Alloy preparation Small billets of the alloys were prepared by induction melting in recrystallized alumina crucibles, under an atmosphere of argon, followed by casting into split copper molds. During the actual casting operation, the melts were exposed to air for a period of a few seconds. The raw materials consisted of 99.999 percent Al rod and 99.99 percent Mg rod. The silicon was of semiconductor grade 4 purity. Beryllium was CR grade flake having the impurities shown in Table I. To facilitate easy dissolution each piece of Be was coated with a glaze of LiCl/LiF prior to melting of the alloy. Chemical analysis of a large number of alloys has shown that nearly all Li is lost during melting.

The nominal compositions of the alloys were:

1 A1, 0.9% Mg, 0.9% Si 2 A1, 0.9% Mg, 0.9% Si, 0.5% Be Table II contains the actual analysis of the alloys after preparation.

TABLE I.-MAJOR IMPURITIES IN BERYLLIUM CR GRADE FLAKE TABLE II.OHEMICAL ANALYSIS OF ALLOYS (WT. PERCENT) Mg Si Be (1) Al-Mg-Si 0.89 0. 91 2 Al-Mg-Si-Be 0. 07 0.97 0.38

Subsequent to casting and cropping, each billet was cold rolled by 20% thickness reduction and homogenized at 450 C. for one week. In this condition the specimens were composed of equiaxed grains of 0.25 mm. mean size.

Heat treatment conditions All specimens were solutionized at 480i3 C. for 3 hours in a vertical furnace under an atmosphere of argon followed by a direct quench into water at 0 C. The quenching rate was determined approximately using a cylindrical sample of 6 mm. diameter x 36 mm. long with a thermocouple embedded within a blind, axial hole. The thermocouple signal was displayed on an oscilloscope. This specimen cooled to near 0 C. in 1.1 seconds. Therefore, it is concluded that the smaller tensile and strip specimens probably cooled in a time considerably less than 1 second.

Ageing was carried out immediately after quenching. This operation was performed using a silicone oil bath, temperature controlled to :Mr" C. Nearly all work was performed at 240 C. for periods of up to 2 hours.

Mechanical property determinations Dumbbell shaped tensile bars were machined from the homogenized alloy billets. These had a gauge section of 30 mm. length and 3 mm. diameter. An exteusometer of 12.5 mm. gauge length was used for strain measurements in a standard 10,000 lb. Instron machine. Throughout these tests a cross-head speed of 1.25 10 mm. per minute was utilized.

For the microstrain measurements flat tensile bars were milled to shape. These were of 50 mm. length X 3 mm. thick x 9 mm. wide. Strain determinations were made using a Tuckerman optical strain gauge which was clamped onto the specimens by means of a light-tension spring. Two sheets of plexiglass were fitted to the front and rear of the Instron frame for the purpose of reducing draughts, which would otherwise lead to spurious strain readings, around the specimens.

After positioning in the tensile machine, the system was left for 30 minutes to permit stabilization whereon a series of load-unload cycles was performed with progressively increasing stress amplitude. The tests were continued until a permanent strain of 2x10 was obtained.

The cross-head motion throughout these measurements was 1.25 X mm./min.

The results are shown in FIGS. 2-4.

What is claimed is:

1. An aluminum alloy consisting essentially of about 0.6 to 3.6% by weight, magnesium; about 0.5 to 1.5% silicon, and about 0.2 to 0.5% beryllium, balance aluminum 2. An alloy according to claim 1 wherein the magnesium to silicon content is magnesium rich.

3. An alloy according to claim 1 wherein the magnesium to silicon content is silicon rich.

4. An alloy according to claim 3 wherein the atomic ratio of magnesium to silicon in said alloy is not more than 2 to 1.

5. An alloy according to claim 2 wherein the atomic ratio of magnesium to silicon in said alloy is about 2 to l.

6. An alloy according to claim 4 wherein no significant amount of Al Mg is present.

7. An alloy according to claim 4 wherein said magnesium and said silicon constitutents combined to form magnesium silicide with a trace of free silicon remaining in said aluminum alloy.

8. An alloy according to claim 7 wherein said trace of said free silicon is not more than 1.0 percent by weight of the alloy.

9. An alloy according to claim 7 wherein said trace of said free silicon is not more than 0.5 percent by Weight of the alloy.

10. An alloy according to claim 8 in which the silicon content is not more than about 1.0% and the magnesium content is not more than about 1.0% by weight.

11. An alloy according to claim 10 in which the magnesium content is about 0.9 to 1% by weight.

12. An alloy according to claim 11 in which the silicon content is about 0.9 to 1% by weight.

13. An alloy according to claim 12 in which the beryllium content is about 0.5% by weight.

14. A method of heat treating an aluminum alloy containing about 0.6 to 3.6% Mg, about .5 to 1.5% Si and about 0.2 to 0.5% Be, balance aluminum comprising:

providing an alloy having the said composition;

solution treating said alloy at a temperature above 375 C. for a time suificient to dissolve a substantial amount of the Mg and Si and at least some Be in solid solution so as to retain at least a portion of said Mg and Si in solid solution; quenching said alloy;

ageing said alloy at a temperature of from to 400 C. for a time suificient to increase the yield and tensile strength of said alloy.

15. A method according to claim 14 in which the alloy is melted and cast with a cooling rate of at least 3 C./ sec. to obtain a fine Mg Si dispersion.

16. A method according to claim 14 in whch the alloy is aged at 200 to 220 C. for 1 to 4 hours.

17. A method according to claim 14 in which the solution treatment is carried out at 400 to 500 C.

18. A method according to claim 14 wherein the magnesium to silicon content is magnesium rich.

19. A method according to claim 14 wherein the magnesium to silicon content is silicon rich.

20. A method according to claim 14 wherein the atomic ratio of magnesium to silicon in said alloy is not more than 2 to 1.

21. A method according to claim 14 wherein the atomic ratio of magnesium to silicon in said alloy is about 2 to 1.

22. A method according to claim 14 wherein the silicon content is not more than about 1.0% and the magnesium content is not more than about 1.0% by weight.

23. A method according to claim 14 wherein the magnesium content is about 0.9 to 1.0% by weight.

24. A method according to claim 23 wherein the silicon content is about 0.9 to 1.0% by Weight.

25. A method according to claim 24 wherein the beryllium content is about 0.5% by weight.

References Cited UNITED STATES PATENTS RICHARD O. DEAN, Primary Examiner US. Cl. X.R. 148-325, 159 

